I now have an Adafruit Trinket Pro (5v) in house for the first prototype. For the time being, blue LED's are being used for the display until an order of multicolor LED's arrives. Today, code for timing function was completed and downloaded. Initial tests show that the code works successfully as a fixed timer for pacing the inhale/exhale breath cycle.
The next step is to code a function that allows information be fed to the Trinket about frequency of his/her current breathing. The user establishes a baseline by pushing a button at the completion of each exhale. The software will calculate current breathing frequency and begin pacing at that frequency. Then the pacing will gradually slow until it matches the target frequency of approximately 6 breaths per minute. This is an important feature of the project. Users often find it hard to slow their breathing to match the target frequency. Custom pacing beginning at the user's baseline is the key to comfortably attaining the very slow breathing rate that produces optimal relaxation.